The present invention relates to noise reducing resonator devices having a relatively small structural volume which is variable and which has a high admittance. Such devices are suitable for reducing the noise pollution in the air and other gaseous, vaporous and liquid media.
The reduction of noise is part of the protection of our environment and constitutes a foremost problem especially with regard to the reduction of noise in workshops, offices, and the like. Many possibilities are available for the noise reduction. However, economical and technical considerations frequently prevent the use of known devices for the intended noise reduction purpose.
It has been suggested to reduce noise by means of destructive interference, please see "Journal of Sound and Vibration", 1970, Nr. 2, pages 223 - 233, by Czarnecki. According to this prior art it is suggested using so-called Helmholtz resonators located near a source of noise which excites the resonators to oscillate in phase opposition thereby contributing to an interference quenching of the noise. This effect may be interpreted as a mismatching of the radiation resistance due to the resonators. Yet another interpretation based on multi-pole analysis suggests that the source of noise which originally operates as a monopole, is transformed into a pole of higher order having a lower operational efficiency.
Helmholtz resonators are frequently used in connection with sound absorbers or dampers. The circuit arrangment may be a series connection or a parallel connection, whereby sound absorbing or damping and sound insulation may be accomplished. As such, Helmholtz resonators are simple and very efficient structural elements. However, the disadvantage resides in the fact that these resonators require a large structural volume in the lower frequency range. On the other hand, a Helmholtz resonator has a relatively narrow frequency band or range in which it operates efficiently. Thus, due to the required volume it is not always possible to utilize several, differently tuned Helmholtz resonators.
On the other hand, well known mechanical resonators, for instance, plates oscillating along with the noise source have an input impedance which is too high, in other words, their admittance is too low so that they are efficient only where a large surface area may be exposed to the sound or noise to be reduced.